Proofpoint Essentials filters email for viruses and spam, along with some admin/user defined rules. The service is cloud based (no on premise hardware) and acts as a go-between the internet and your mail server. This is accomplished by modifying the mail exchange (MX) record on a domain at the hosting level. Once the mail passes through the Proofpoint Essentials network, it then passes back to your mail server via server destination settings in our management interface. As the messages are passed to your server, POP and IMAP functionality will continue to work as normal. No large changes to your mail server change in this aspect. Normally when a customer's domain is set up for using Proofpoint Essentials, the MX records point to Proofpoint Essentials as "SMTP Destinations"... and the same apply for customers using POP/IMAP. Usually there's nothing more to it, but do take note of the following two points:

We recommend confirming that the ISP will not disable the POP/IMAP accounts because the MX records have moved. It does happen, unfortunately.

Hosted email providers generally will not lock down their firewall as per our recommendations, and spammers will still be able to deliver email directly to the ISP server bypassing Proofpoint Essentials. There are ways around this - please refer to the knowledgebase article on firewall lock down.

Terminology:

SMTP: The protocol by which email is sent from one server to another over the internet.

POP: An older protocol for accessing hosted email, most frequently used to download emails to a local mailbox before managing the emails locally.

IMAP: A newer protocol for accessing and managing hosted email, most frequently used to keep and manage emails on a remote server.